In the driving devices of the main shaft of a machine tool and a vehicle which are to be driven by an AC variable frequency power source, a winding switching method has been employed as means capable of obtaining a sufficiently great torque in a low speed region and carrying out an operation in a high speed region.
As an example, a star-delta switching method shown in FIG. 6 has widely been used practically for the driving operation of the main shaft of a machine tool. In FIG. 6, 22 denotes a power source, 16 to 21 denote a diode constituting a three-phase full-wave rectifying bridge, and 15 denotes a smoothing capacitor. 14 denotes a converter section for converting the AC power source 22 into a DC power source. Terminals TP and TN are DC output terminals of the converter section 14, and serve as inputs of the inverter section 1. 2 denotes an AC motor, T1 to T6 denote a terminal to be used in switching, and 3 and 4 denote a switch such as an electromagnetic contactor. When the switch 4 is opened and the switch 3 is closed, a star connection is obtained. When the switch 4 is closed and the switch 3 is opened, a delta connection is obtained. N1 denotes a neutral point. In the low speed region, a star (Y) connection is selected to apply a sufficiently high voltage so that a great torque can be obtained for the same current. Since the impedance of the motor is increased in proportion to a frequency, the current flow might be impeded in the high speed region in which the frequency is increased. By selecting a delta (A) connection having a low impedance, therefore, the current flow can be conducted easily.
In FIG. 7, two sets of star windings are switched in series and parallel. A switch 5 is closed to connect the windings in series at a low speed, and switches 6 and 7 are closed to connect the windings in parallel at a high speed. Consequently, the same advantage as that of FIG. 6 can be obtained. Furthermore, FIG. 8 is obtained by simplifying the circuit in FIG. 7. When a switch 8 is closed, all windings are utilized equivalently to a series connection. When a switch 9 is closed, a part of the windings is used so that a characteristic corresponding to a parallel connection in FIG. 7 is obtained. In this case, the residual windings are not used but in an idle state. For this reason, a current density is increased to be a double of that in FIG. 7. However, the number of winds for creating a magnetic flux is equal. Consequently, an induced voltage and a torque characteristic are basically equivalent to those in the parallel connection.
While switching in two stages is carried out in all the examples described above, a method of carrying out switching in three stages, and furthermore, a more precise control has been disclosed in U.S. Pat. No. 3,037,471.
In all the examples described above, it is assumed that switching is carried out by a switch having a mechanical contact. There has been made a proposal for reducing a dead time for switching with the operating time of the switch. FIG. 9 has been disclosed in JP-B-7-99959 by the applicant, in which two sets of inverters are combined to switch a star connection and a delta connection without contact by a change in a method of controlling each inverter. FIG. 10 has been published in IEEE Transactions on Industry Applications, Vol. 32d No. 4, July/August, 1996, pp. 938-944. Two sets of windings having different specifications which are provided in the same motor are driven by two inverters and a combination of respective current vectors is changed, thereby switching double-pole and four-pole characteristics.
Moreover, U.S. Pat. No. 2,742,800 has disclosed a method of applying circuits connecting, as a switching unit, a semiconductor control unit and a diode for blocking a reverse voltage in series which are connected in antiparallel based on the circuit in FIG. 8.
In the methods shown in FIGS. 6, 7 and 8 and the technology in the U.S. Pat. No. 3,037,471, the switching is carried out with a switch having a contact. Accordingly, it is necessary to take a time for such a mechanism operation as to turn on and off the contact. Moreover, it is desirable that a current should be once blocked on an inverter side to carry out so-called non-current switching in consideration of the lifetime of the contact. When these operating times are summed, the dead time that cannot be disregarded (usually, several tens milliseconds) is generated. For example, the dead time influences the quality of an end product in a driving device for the main shaft of a machine tool, and furthermore, influences a feeling of riding in a driving device for a vehicle. A limited contact lifetime itself is also a disadvantage which cannot be permitted.
In the methods of FIGS. 8 and 9 and the U.S. Pat. No. 2,742,800, the switching is carried out by opening and closing through the semiconductor unit or a change in a control mode. Consequently, the problem of the operating time can be improved. Since the number of active semiconductor units to be required is large one, a cost might be a factor in impediment to practical use.
In the methods of FIG. 8 and the U.S. Pat. No. 2,742,800, furthermore, a voltage induced into the residual winding portions is added to a supply voltage and a high voltage is applied to an unused terminal when a power is supplied to the neutral point of the winding. Therefore, it is necessary to intensify an insulation.